Abstract: Quickly increased complexity, openness, interconnection and interdependence have made computer systems more vulnerable and difficult to protect from malicious attacks. Network intrusion detection system plays a vital role in today`s network. In this system using Genetic Algorithm (GA), crossover, mutation and other related techniques to used to detect network intrusion system. As the transmission of data over the Internet increases, the need to protect connected systems also increases. Although, the field of IDSs is still developing, the systems that do exist are still not complete, in the sense that they are not able to detect all types of intrusions. Some attacks which are detected by various tools available today cannot be detected by other products, depending on the types and methods that they are built on. Network Intrusion Detection Systems based on crossover and mutation is the latest technology used for this purpose. The network intrusion detection system should be adaptable to all type of critical situations arise in network. This is helpful for identification of complex anomalous behaviors. This research is focused on the TCP/IP network protocols.